ADF4158 clk1 and clk2 setting

I am using ADF4158 for the source of my FMCW radar, hence I set the frequency sweep with the ramp. I would like to know if there is a certain rule to set the clk1 and clk2 parameters or they can be set freely, just to obtained desired ramp period. (e.g. clk1 = 18 and clk2 = 9)